1. Field of the Invention
The present invention relates to an electron beam melting method for metallic materials, in particular, relates to a melting method which can restrict increase of oxygen concentration in produced metallic ingots by producing metallic ingots by an electron beam melting method, thereby the ingots being widely useful in technical fields such as semiconductor materials which require severely restricted oxygen content.
2. Description of the Related Art
In an electron beam melting method for metallic materials, an electron beam is emitted onto a metallic material in a vacuum to melt the material, the melted material is charged into a crucible cooled by water for solidification. The open bottom of the crucible is closed by a starting block, which is moved downwardly so as to continually extract the coagulated metallic material to produce an ingot. The electron beam melting method is widely used for producing high melting point materials such as W, Nb, and Mo, active materials such as Ti, and highly pure Cu.
When an electron beam is emitted onto the material and the temperature of the material is increased, heat radiates from the material. A large amount of moisture and gases in the air adheres to the inner walls and surfaces of several parts in a melting furnace before the melting of the metallic material, and a certain portion of the adhered moisture and gases remain in the melting furnace even if the interior of the furnace is evacuated. The remaining moisture and the gases are separated from the surfaces of the inner walls and the parts and are discharged into the interior of the melting furnace. As a result, portions of the discharged moisture and gases are absorbed into the melted material, thereby increasing the oxygen concentration.
The adhered moisture and gases are intensely discharged at the beginning of the melting, and the amount thereof is gradually reduced as the melting is proceeded. When the amount of radiated heat is large, the adhered moisture and gases are intensely discharged, and the amounts thereof are reduced in a short time. Therefore, in a case of electron beam melting of materials with strong affinity for oxygen, such as Ti and Zr, the oxygen concentration may exceed 50 ppm in a portion produced at the beginning of the melting. Such a portion with high oxygen concentration deteriorates workability and reliability and may be cut and removed, thereby decreasing the yield efficiency.
In order to restrict increase of the oxygen concentration, Japanese Patent First Publication No. 9-31559 discloses pre-melting the same material as the melting material. However, the publication only discloses a pre-melting method using a water cooled tray type melting vessel (in section [0024]), and the problems such as decrease of the yield efficiency and increase of the oxygen concentration remain unsolved.